Currently, the most widely used engines are the Diesel and Otto spark ignited (SI) engines. While these engines offer remarkable performance at a relatively low cost, their basic design of over one hundred years ago is in need of serious improvement in the areas of thermal and pumping efficiency, emissions, and fuel economy. Refinement of the design of both engine types has improved the performance of these engines in recent decades. However, major improvements in the future are unlikely as this mature design is restricted by basic design limitations.
One serious limitation is the volume of air which can be pumped through the engine. Pumping volume has historically been increased through incorporating larger intake and exhaust valve openings and by supercharging. The total possible valve opening size of the intake and exhaust valves has been effectively reached in recent years with the use of multiple intake and exhaust valves per cylinder. The valve opening limit is imposed by the physical dimensions of the head and the need to have the intake and exhaust valves, and the ignition device located in the same physical proximity.
Another important limitation of conventional engines is thermal efficiency. While the diesel engine offers higher thermal efficiencies than the SI engine, both engine types use dedicated combustion chamber designs for each cylinder. Thus, the cylinder walls and combustion chambers transfer and lose heat thereby reducing thermal efficiency. Adding to this problem is the use of single stage combustion systems which have large temperature differences between the reactants of the fuel and air and the products (exhaust). This increases thermodynamic entropy.
In recent years, the importance of fuel economy has increased with the dramatic increase in fuel prices. Although diesel engines are more fuel efficient than SI engines, diesels suffer from lack of power and produce more noise and vibration than SI engines. This has greatly reduced the acceptance of diesel technology in the US and the fuel savings they provide.
Emissions, especially NOx, CO2, and soot are problematic in conventional engine technology. Diesel engines are prone to producing NOx and soot while SI engines have elevated CO2 emissions which is a major contributor to global warming. Since, the effects of global warming are of increasing concern, CO2 emissions need to be addressed as well as all other emissions to meet increasingly stringent emissions regulations. Reductions in CO2 emissions are best accomplished by burning less fuel, or put another way, increasing fuel mileage. Increasing fuel mileage would then require vehicle weight reduction, reducing output, increasing engine efficiency, or some combination of these. Reducing output is the least costly option, but consumers of light duty vehicles have historically shown a distinct preference for high output vehicles as well as vehicles with many options which increases weight. This presents a real dilemma for manufacturers of these vehicles since conventional technology cannot simultaneously increase output, weight, fuel economy, and emissions in a cost effective manner.
Long-term, the best way to reduce emissions and fuel consumption without decreasing output is to increase the overall engine efficiency. Current developments in conventional technology show promise in emissions reduction with new combustion techniques such as Homogeneous Charge Compression Ignition, Low Temperature Combustion, Stratified Charge, and Stratified Charge Radical Ignition. However, these techniques make little or no increase in engine efficiency and some cannot operate over all load and speed ranges.
Other engine designs also have their own inherent problems. Two stroke versions of the Diesel and Otto engines have considerably higher power-to-weight ratios, but due to inefficient scavenging are not as thermally efficient and have considerably inferior emissions characteristics. Rotary engines with their large combustion surfaces also are not as thermally efficient and exhibit higher emissions although they posses high power-to-weight characteristics. Finally, rotary valve engines can exhibit high power-to-weight ratios, but historically have been plagued by sealing problems leading to either excessive seal wear or oil consumption.
However the rotary valve engine has some intriguing characteristics if it could overcome the sealing problem. First, consider the typical rotary valve engine having one rotary valve per cylinder. The rotary valve engine can be constructed without the need to have hot exhaust valves in the combustion chamber. This allows the compression ratio in SI engines to be increased producing both increased thermal efficiency and output. Further, rotary engines can operate at significantly increased RPMs to produce even greater output. In addition, rotary valve engines would be expected to exhibit emissions similar to four stroke conventional engines, but with the right combustion chamber and valve geometry, could outperform them. Finally, rotary valve engines require fewer parts than conventional engines which should increase reliability while decreasing the manufacturing cost. These characteristics may explain why interest in them remains so high.
Considering these advantages, closer inspection of the seal problem is warranted. One of the more promising designs was the Aspin rotary valve engine using a conical shaped rotary valve seated against a ring seal above it. As was typical of most rotary valve engines, combustion pressures were exerted against the seals to increase combustion chamber sealing. The resultant pressures on the seals required them to be well lubricated to prevent excessive seal wear or rotary valve seizure. However, adequate lubrication of the seals produced excessive oil consumption while reducing the lubrication produced excessive seal wear. No solution was ever found to this problem.
Another promising design was the 1950 era NSU single cylinder rotary valve motorcycle engine. This was a vertical disc valve engine using a ring seal below the valve to seal the combustion chamber. This seal also exhibited excessive wear from exposure to combustion pressures. A later version using a single valve serving two cylinders was tested. Due to a slight performance advantage, the NSU engine apparently was a successful race engine but was abandoned in favor of development of the Wankel rotary engine.
Contrary to the other engines discussed which limit efficiency gains due to basic design constraints, the rotary valve engine seal problem is due to the failure to find a technical engineering solution. One answer to this technical problem may have already been found by Coates International Ltd. This spherical rotary valve design apparently has overcome the seal problem but reliability of the engines is unknown. It should be pointed out; the Wankel rotary engine originally also was unsuccessful due to excessive seal deterioration. After the demise of NSU due to their inability to solve this problem, it took the resources of Toyo Kogyo (Mazda) to finally solve the remaining engineering issues to make the Wankel rotary engine successful. With normal maintenance, the seal systems of present Wankel rotary engines should last approximately 300,000 miles. Given the eventual success of the Wankel rotary engine, one has to believe that if the NSU rotary valve engine would have received the same resources, the seal problem would have been solved to produce a successful rotary valve engine.
All three rotary valve engines cited claim performance advantages. However, none is independently documented. The Coates engine also claims increased volumetric efficiency, but this claim is suspect due to no claim of actually increasing the valve opening areas, although some increased pumping efficiency is likely at high engine speeds. With respect to thermal efficiency, some gain in SI rotary valve engines is likely due to increased compression ratios. Additional thermodynamic gains are unlikely since all the rotary valve engines cited utilize combustion chamber geometries similar to conventional poppet valve engines.
The internal combustion engine is a heat engine. As such, increases in engine efficiency will need to address the areas where there are the greatest heat losses or more technically, the greatest thermodynamic entropy. These are the combustion process which generates thermodynamic irreversibility and the transfer and loss of heat to various combustion surfaces. Thermodynamic irreversibility is created by the combustion reactions transforming fuel energy into molecular motion and heat. This heat energy is unavailable for work and is lost. Typically, reducing the thermodynamic irreversibility only serves to increase exhaust heat losses. Therefore, significant increases in thermodynamic efficiency will require reducing heat losses to the cylinder and combustion chambers, reducing thermodynamic irreversibility and recapturing exhaust heat energy to produce useful work. This implies that significant energy efficiency improvements will necessitate substantial changes in engine architecture.